Among the materials that impart detergency to lubricating oils to keep internal engine parts clean and reduce sludge formation in the oil are overbased detergents, particularly calcium sulfonates. These sulfonates are known to be useful as additives for lubricating oils, particularly as a crankcase engine oil for internal combustion engines.
Overbased calcium sulfonates are generally produced by carbonating a mixture of hydrocarbons, sulfonic acid, calcium oxide or calcium hydroxide and promoters such as methanol and water. In carbonation, the calcium oxide or hydroxide reacts with the gaseous carbon dioxide to form calcium carbonate. The sulfonic acid is neutralized with an excess of CaO or Ca(OH), to form the sulfonate. The prior art known processes for overbasing calcium sulfonates produce high alkaline reserves of TBN of 300 to 400 mg KOH/gm or higher, which enables the formulator to use lower amounts of additive while maintaining equivalent detergency to protect the engine adequately under conditions of high acid formation in the combustion process.
The calcium carbonate component of the overbased calcium sulfonate forms the core of a calcium sulfonate micellar structure. The calcium carbonate is either in the amorphous and/or one or more of its crystalline forms particularly, calcite.
Dispersions of non amorphous or so called crystalline calcium carbonates are generally very cloudy and highly viscous materials. These are prepared by a variety of processes such as disclosed in U.S. Pat. No. 3,242,079, U.S. Pat. No. 3,376,222, U.S. Pat. No. 4,560,489, U.S. Pat. No. 4,597,880, U.S. Pat. No. 4,824,584 and U.S. Pat. No. 5,338,467. They find limited application as rust preventative coatings, rheology modifiers, extreme pressure (EP) metal working formulations and greases. Although colloidal dispersions comprising crystalline calcium carbonate are widely used in formulating greases, they generally cannot be used in formulated engine oil lubricants because it does not give acceptably transparent formulations and it provides very high viscosities. Calcite carbonates, however, have desirable antiwear properties so that it would be advantageous to use such materials in engine lubricating oils.
The lubricating oil art, particularly as directed to automotive crankcase and other engine oils, mandates a clear or substantially haze free product for requisite consumer aesthetics and acceptance. This need precluded the use of prior art detergents with haze producing crystalline calcium carbonate.
Papke, et al., U.S. Pat. No. 4,995,993, recognized that large micellar crystalline calcium carbonate structures caused haze, and overbased sulfonate products containing crystalline calcium carbonates are always undesirable and therefore crystallization was to be avoided at all costs. See col. 4, lines 39-42 of U.S. Pat. No. 4,995,993.
In "Colloidal Anti-wear Additives 2. Tribological Behavior of Colloidal Additives in Mild Wear Regime," J. L. Mansot, et al., Colloids and Surfaces A: Physico Chemical and Engineering Aspects, 75 (1993), pp. 25-31, it is indicated that for certain forms of an overbased sulfonate containing an amorphous calcium carbonate core, when in a 2% by weight dispersion in dodecane, and subjected to metallic friction surfaces, the calcium carbonate forms a polycrystalline film adherent to the metallic friction surfaces, which resultantly provides anti-wear protection. Mansot, et al. thereby directed one to providing an overbased calcium sulfonate with an amorphous micellar structure which would then, under a mild wear regime, undergo transformation to microcrystalline agglomerates through an amorphous intergranular phase. Mansot, et al., in this manner, further confirmed the direction of the prior art to providing amorphous calcium carbonate micellar dispersion overbased calcium sulfonate detergents.
In WO 0004113, a process for producing soluble overbased calcite-containing detergents are described which are suitable for use in engine oil formulations. However, for many purposes it would be more advantageous to be able to use a conventionally prepared calcite-containing detergent in an engine oil formulation.